There is a huge power asymmetry between Hezbollah (and Hamas etc) and Israel/USA. The way to offset that is via asymmetric and guerrilla warfare, but they cannot respond to targeted assassinations / drone strikes in the same way, it's simply not possible.
It is indeed a huge escalation, though, and Seyed Nasrallah previously promised a strong response to any Israeli assassinations in Lebanon, so we will for sure see a strong response by Hezbollah. Although to go to full war over this would be very stupid, and Hezbollah are not stupid like others, so they will try to respond in a calculated way that restores deterrence without escalating to a full war (which would entirely obliterate large parts of Lebanon, like we see Gaza being turned to dust, a fate no Lebanese wants).